One of the three "Serpent’s Heads that graced the Bronze Serpent Column that originally formed the base of a "trophy" that was dedicated to the god Apollo after the victory of the Greeks over the Persians in the battle of Plataea in 478/477 B.C.
It originally was placed at the head of the "Sacred Way," opposite the Temple of Apollo, at Delphi (Greece). Constantine brought the "trophy" from Delphi to his New Rome (Constantinople/Istanbul) after he had established his capital there. In the ninth century it was move to the spine of the Hippodrome.
For a view of the base of the statue Click Here.